Regarding your first question, I don't know of any instance in which irradiation was used to select for a beneficial trait in humans. I italicized 'used' because I think that you could argue that naturally occurring sources of irradiation have led to the selection of traits that protect us from radiation damage.
That might sound like a bit of a circular argument, but keep in mind that in the case of the rice you mention, the discovery of beneficial traits came at the expense of a lot of dead rice that suffered deleterious effects of radiation. Unfortunately, the deleterious effects are a lot more likely to occur than than any really useful ones.
That leads to your second question. Conceivably, radiation could lead to a beneficial mutation. However, there are some pretty big caveats with that. First, in order for it to be beneficial, this hypothetical mutation would have to occur very early on in development, otherwise it would be limited to, at best, a handful of cells, probably in your skin, which would just get sloughed off anyway. So the second caveat is that in order for such a beneficial radiation-induced mutation to occur in the right place at the right developmental time, it would have to penetrate all the layers of your mother's genetic material that separate your fetal cells from the outside world without losing the energy needed to damage your DNA.
I couldn't tell you what sort of radiation might be capable of pulling that off with what probability of success, but my guess is that it's really unlikely.
